Alkylcarbenium Ion Concentrations in Zeolite Pores during Octane Hydrocracking on Pt/H-USY Zeolite, Catal. Lett.

Hydroconversion of octane over platinum loaded acid zeolites was simulated using a single-event microkinetic model. Significantly different values for the alkene standard protonation enthalpies, i.e.,) 59.2 and - 94 kJ mol(-1) for the charging of secondary and tertiary carbon atoms respectively, were obtained. This difference is in favor of a carbocationic nature of the reactive intermediates on the acid sites rather than surface alkoxides. The concentration of alkylcarbenium ions on a Pt/H-USY catalyst resulting from protonation of alkenes in n-octane hydrocracking was calculated. It was strongly dependent on pressure and temperature. At a reaction temperature of 506 K, a total pressure of 0.45 MPa and H-2/HC molar ratio of 13.13, the concentration of alkylcarbenium ions corresponds to 15% of the total acid site concentration. At higher total pressures this percentage is lower and can be assumed to be negligible. The presence of a finite alkylcarbenium ion concentration in the zeolite pores results in a reduction of the free space available for physisorption of alkanes. Refined kinetic models are obtained when including this effect. Depending on the nature of the zeolite, alkylcarbenium ion concentrations can be significantly different owing to differences in alkane physisorption and alkene protonation. Literature data were rationalized using the refined kinetic model.